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Astron. Astrophys. 322, 493-506 (1997)

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A period analysis of the optical line variability of [FORMULA] Cephei: evidence for multi-mode pulsation and rotational modulation *

J.H. Telting 1, 2, C. Aerts ** 3 and P. Mathias 4

1 Astronomical Institute Anton Pannekoek, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Isaac Newton Group of Telescopes, Royal Greenwhich Observatory, Apartado 321, E-38780 Santa Cruz de La Palma, Spain
3 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, B-3001 Heverlee, Belgium
4 Observatoire de la Côte d'Azur, Département Fresnel, U.R.A. 1361, BP 229, F-06304 Nice Cedex 4, France

Received 8 July 1996 / Accepted 31 October 1996

Abstract

We investigate the optical line-profile variability of the prototype of the [FORMULA] Cephei stars. From more than 600 spectra, with a typical signal to noise ratio of over 200 and high spectral resolution, we deduce that the variations show periodicity with at least five different frequencies. We find variability at the previously known frequencies [FORMULA] =5.25 cycles/day (radial pulsation) and [FORMULA] =5.38 c/d and [FORMULA] =4.92 c/d. Additionally, we report two new frequencies [FORMULA] =5.08 c/d and [FORMULA] =5.42 c/d. These five frequencies are present in periodograms of the velocity moments of all of the absorption lines considered (Si III [FORMULA] 4552, 4567, 4574 and O II [FORMULA] 4591), and also in periodograms of the intensity variations in these lines.

The variations at frequencies [FORMULA] and [FORMULA] can be successfully modelled by a radial and a non-radial ([FORMULA] =2 or [FORMULA] =1, m =1) pulsation, respectively.

Following results of studies of the UV resonance lines of [FORMULA] Cep, we interpret the frequency spacings [FORMULA] - [FORMULA] [FORMULA] 0.33 c/d, [FORMULA] - [FORMULA] [FORMULA] 0.17 c/d, [FORMULA] - [FORMULA] [FORMULA] 0.17 c/d as multiples of the rotation frequency of the star.

We discuss the influence of the presence of a (surface) magnetic field on the apparent modulation of the line-profile variability of the radial pulsation mode.

Key words: line: profiles – stars: early-type; oscillations; rotation – stars: individual: fi Cep

* Based on observations obtained at the Observatoire de Haute Provence, France
** Senior Research Assistant, Belgian National Fund for Scientific Research

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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